Cellulose fiber cans



y 1967 J. B. FELTON, JR., ET AL 3,322,318

CELLULOSE FIBER CANS Filed Nov. 12, 1965 FIG. 4

FIG. 7

United States Patent Ofiice 3,322,318 Patented May 30, 1967 3,322,318CELLULGSE FEBER (IANS Joseph B. Felton, .lr., Mount Pleasant, John E.Turner, North Charleston, and Archie D. Duncan, In, and William J.Funderhurk, Charleston, $.(L, and Roger Bart, Weston, Conn, assignors toWest Virginia Pulp and Paper Company, New York, N.Y., a corporation ofDelaware Filed Nov. 12, 194%, fier. No. 322,705 6 Claims. (CL 229-65)This invention relates to containers which can be employed in thepackaging of thermally processed foods and more particularly relates tosuch containers constructed of thermosetting resin-impregnated non-wovencellulose fiber webs and to methods of preparing these containers.

The familiar tin can has proved to be a very effectual and economicalpackage for a great variety of products. In recent years, however, otherless costly containers have been developed which have replaced the tincan for the packaging of certain of these products. These newlydeveloped containers have been constructed primarily of paper orpaperboard in combination with metal foil or thermoplastic resincoatings, and are now being employed widely for packaging suchdiversified products as motor oil, frozen fruit juices, and refrigeratedbakery goods. While the tin can has lost some markets to fiber cans, thetin can remains essentially unchallenged in its use for pack-agingthermally processed foods. The less expensive fiber cans have hithertobeen incapable of withstanding the rigorous conditions involved inthermal processing and the only competition at all with the tin can inthis area of use has been from the more expensive glass containers andaluminum cans.

Thermal processing of foods involves cooking of the food product afterit has been sealed in the can or container, so as to destroy allorganisms that might cause spoilage. The exact conditions employed inthermal processing vary considerably depending primarily upon the beingcanned. However, regardless of the type of food being canned, thermalprocessing involves the use of relatively high temperatures in thepresence or" Water or steam, resulting in internal and externalpressures being alternately applied to the can. The temperatures andrelated conditions employed in thermal processing require the use ofcans constructed from materials having much better physicalcharacteristics than are provided by present fiber based cans.

A fairly typical example of conditions encountered in thermal processingis in the canning of peas. The first step is to blanch the peas at atemperature of about 160 F. Peas at this temperature are then packedinto the can with hot Water. While still open, the cans are exhaustedusually by heating in a steam chamber or by passage of a steam jet overthe open end to remove any air. The cans are next sealed and processedin a steam autoclave at 240 F. for varying lengths of time, depending oncan size, to destroy any injurious organisms. At the temperatureemployed in this last step a gage pressure of about 8 p.s.i.g. isdeveloped in the autoclave. As the liquid in the can is heated, analmost equal pressure is built up within the can. The net result is thatvery little pressure differential exists while the can remains in theautoclave. When the sealed can is removed from the autoclave, however,the pressure on the outside of the can is rapidly decreased toatmospheric while the contents of the can, still being at about 240 F.maintain the internal pressure of about 8 p.s.i.g. As the can cools theinternal pressure in the can decreases, finally becoming zero, andpasses to a negative interior gage pressure, or vacuum, due to thecooling of the contents to a temperature below that at which the canswere sealed.

The can structure must consequently be able to withand the effects ofhigh temperature, high humidity and moisture, pressure and vacuum. Thecharacteristics of paper, paperboard, and similar non-woven cellulosefiber webs are such that both high temperature and humidity or waterhave a significantly detrimental effect on the strength propertiesresulting in severe loss of ability to withstand the pressure andvacuum. To our knowledge, no presently available fiber can constructionis consistently capable of performing satisfactorily under theconditions of thermal processing. While it is conceivable that, bygreatly increasing the quantities of materials employed in the availablefiber cans and by encapsulating the cellulose web so as to eliminate allcontact of steam or water with the web, a can could be made which wouldprovide satisfactory service, such cans would be wholly impractical dueto their great bulk or high cost.

The primary object of the present invention is to provide a canconstructed of non-woven cellulose fiber web material which canpractically be employed for packaging of thermal processed foods andwhich can compete with the common tin can.

Other objects will become apparent from the following disclosure.

We have found that a can capable of being employed in the packaging ofthermal processed foods and in substantial all other packaging uses inwhich the tin can is currently employed can be constructed from pressurecured thermosetting resin-impregnated non-woven cellulose fiber webs.

In the practice of this invention, non-woven cellulose Webs, such aspaper and paperboard, are impregnated with a thermosetting resin and theresin cured under pressure in the web structure. The resultant thinsheets of i thereafter be cut to be made of the same resin-impregnatedweb material as that employed in the can body, tinplate, aluminum, orany other suitable materials such as high temperatureresistant moldedplastics. While, in general, the basic steps employed in converting thethin sheets of resin-impregnated web into a can body are similar tothose employed in making cans from tinplate, the great diiferences inthe properties of the impregnated webs used in this invention comparedto those of tinplate require that significantly different methods beemployed in conducting these basic steps, as will be evident from thedisclosure hereinbelow.

The non-woven cellulose fiber webs employed in this invention possesscertain characteristics: which would apparently make them totallyunsatisfactory for use under the conditions involved in thermal processpackaging. Cellulose fibers derived from any source, whether they arethe naturally occurring pure fiber of cotton or the pulp obtained bystringent chemical treatment of wood, are seriously effected by bothheat and moisture; the two conditions which are characteristic ofthermal processing. Non-woven cellulose fiber webs ing mechanisms areseriously affected by moisture and/ or heat. For example, paper can loseup to of its strength by soaking it in water and may lose about 30% ofits strength by subjecting it to an environment at F. In addition, thedetrimental efiects of both moisture and temperature are greatlyincreased when they occur in conjunction with one another as isencountered during thermal processing.

In spite of these inherent disadvantages in the characteristics ofnon-woven cellulose webs, these Webs when combined with thermosettingresins in accordance with the principles of this invention cansatisfactorily be utilized in making cans for thermal process packaging.

A wide variety of thermosetting resins may be employed in the practiceof this invention. Satisfactory resins include the allyl resins whichare based upon such diallyl prepolymers as diallyl phthalate or diallylisophthalate and which are cured to the thermoset state with peroxidecatalysts, the amino resins (excluding ureaaldehyde resins which lackthe required resistance to moisture) which are based upon the reactionof a polyamine such as melamine and an aldehyde such as formaldehyde,the epoxy resins which are based upon the polymerization of prepolymershaving a plurality of oxirane groups, such as the diglycydyl ether ofbisphenol, under the influence of cross-linking agents or catalysts suchas acids or amines, the urethane resins which are based upon thepolymerizing reaction of polyisocyanates with compounds having aplurality of active hydrogens such as the polyhydroxy polymers ofethylene or propylene glycol or of polyhydroxy and polybasic acidiccompounds, the polyester resins including the oil modified polyestersgenerally referred to as alkyd resins, which are based upon thecrosslinking of copolymers, formed by the reaction of a polybasic acidand a polyhydric -alcohol, through unsaturated groups in the copolymergenerally by vinyl compounds, the phenolic resins which are based uponthe reaction of a phenolic compound with an aldehyde such asform-aldehyde, those thermosetting polycarbonate resins (as contrastedto the thermoplastic polycarbonate plastics) which are based upon thereaction between unsaturated and aliphatic dehydr-oxy compounds withphos gene or appropriate phosgene-derived precurors, and the organopolysiloxane based silicone resins.

For practical purposes, the selection of a thermosetting resin for usein this invention will be based primarily upon economic consideration ofthe current cost of the resins and the quantity of resin needed toimpart the necessary properties to the cellulose web. Based upon currentcosts and knowledge, the preferred resins for use in this invention arethe phenolic resins which are relatively inexpensive and can be employedat reasonably low levels. These phenolic resins may be employed atlevels as low as while still producing satisfactory cans, depending uponthe service intended. (As employed herein the percentage of resin is theweight percent of the cured resin solids based upon the total weight ofthe cured resin solids and the cellulose web.) Below 15% the moistureresistance is inadequate to withstand the rigorous conditions of thermalprocessing. Levels of up to about 60% phenolic resin may be employedsatisfactorily; however, above about the 35% level improvement ofproperties is generally insufficient to justify the added cost. A levelof between and phenolic resin has been found to be preferred. The otherthermosetting resins should be employed within the same broad range asthe phenolic resins, i.e., between 15% and 60%, although the preferredrange away be somewhat different.

The resin-impregnated web stock may be constructed of a single ply or aplurality of plies laminated together into a unified coherent sheethaving no sharply defined planes of demarcation throughout its thicknessof either properties or composition. Such a laminated structure does notnecessarily have to be homogeneous, and may have gradual gradations bothin composition and properties throughout its thickness. This is truealso of impregnated web stock made from a single ply.

In order to achieve the necessary properties for withstanding theconditions of thermal processing, it is desirable that the resin bedistributed throughout the cellulose web. The necessity of suchdistribution of the resin will be quite evident when it is recalled thatboth the heat and moisture conditions encountered in thermal processingcause severe deterioration of cellulose fiber-to-fiber bonding. Suchdeterioration of the bond; although it may be in only a relatively smallportion of the web could result in functional failure of the wholestructure. The fact that the resin must be dispersed throughout the webdoes not mean that the resin distribution must be uniform therethrough.It is quite possible to vary the type of resin or to employ reducedamounts of resin in the interior of the web Where the fibers are notsubjected to the effects of heat and moisture to as great a degree as onthe surfaces of the web.

It is essential, in order to obtain web stock capable of withstandingthe thermal processing operation, to cure the thermosetting resin undersufficient pressure to compact the web structure into a substantiallyvoid-free contiguous structure. The pressures necessary to achieve thistype of dense structure, which should have a dry specific gravitygreater than about 1.05, is greatly dependent upon the flow and curecharacteristics of the resin. Pressures as low as 50 p.s.i. are marginalalthough they may be employed with some of the resins at rather highresin percentages. Preferably much higher pressures should be employed,in the neighborhood of 500-1500 p.s.i., especially for phenolic resins.Maximum pressures are listed to those at which compressive degradationof the fibers occurs. The pressures specified need not necessarily beemployed on a constant basis throughout the curing of the resin as it ispossible to reduce the pressure to a much lower level after the initialhigh pressure has caused flow of the resin and has compacted the web.While this second phase lower pressure can beconsiderably lower than theinitial pressure, it should be sufficient to prevent any substantialspring-back of the fibers from their compressed state and should becontinuously applied until the curing of the resin has proceeded to thestage wherein the resin bonding is strong enough to restrain thetendency of the fibers to assume their original configuration in thenon-compressed web.

The temperature employed for curing the resin-impregnated webs will, ofcourse, be dependent upon the specific type of resin employed. Some fewresins, such as the resorcinol-resins and certain of the epoxy andpolyester resins, can be cured at or near room temperature. However,these resins will present obvious problems in pre venting precuring ofthe resin during impregnation of the web and subsequent removal ofsolvent. Most of the thermosetting resins will require curing attemperatures from about 100 to 400 F., as recommended by the resinsupplier.

A relatively simple test has been developed to determine the utility ofcured resin-impregnated web materials in cans subject to thermalprocessing. This test consists of cutting 1" x 3" strips of web stock,subjecting them to saturated steam at 212 F. for 5 minutes in a closedcontainer, and immediately determining the modulus of elasticity infiexure of the material accordingto ASTM 790-61. Because of economicalconsiderations and performance criterion it has been found that the webstock, after the aforementioned steam treatment, must retain at least ofits original modulus of elasticity (as measured after conditioning for 3days in an atmosphere of 50% elative humidity at 73 F.) and after suchsteam treatment the modulus of elasticity should not be less than500,000 p.s.i. Web stocks which do not retain at least 65% of theiroriginal modulus of elasticity do not possess adequate water resistanceproperties to perform satisfactorily in thermal processing applications.Likewise, those web stocks which do not have a minimum modulus ofelasticity of 500,000 p.s.i. after the aforementioned steam treatment,lack adequate rigidity to withstand the vacuums encountered in thermalprocessing applications. It could be pointed out that the 65% retentionof the original modulus of elasticity and the minimum 500,000 p.s.i.modulus of elasticity after steam treatment are minimum requirements forthe web stock; such as for use in the canning of fruit juices, and thatweb stocks which barely meet these requirements will not, in general, besatisfactory under more severe conditions of thermal processing such asencountered in the canning of meats when a temperature of 260 F. isemployed for an extended period of time, and pressure differential ashigh as p.s.i.g. are involved. To operate satisfactorily under the moresevere conditions it would be desirable for the Web stock to retain atleast 90% of the modulus which should not be less than 1,000,000 p.s.i.after the steam treatment.

Due to the anisotropic nature of the properties of nonwoven cellulosefiber webs and the cured resin-impregnated web stock obtained therefrom,the modulus of elasticity in flexure as used herein is the average ofmoduli taken at right angles to one another, preferably in the machinedirection and cross machine direction in the case of paper andpaperboard.

While the impregnated web materials of this invention are capable ofwithstanding the effects of steam, water and temperature without loss ofutility, they are not necessarily completely impervious to water,particularly where the resin content is at the lower end of the rangeset forth hereinabove. Water consequently can be transmitted through thecan walls by wicking action of the cellulose fibers. This permeabilityto water is unrelated to the fact that the impregnated web material isat the same time essentially impermeable to atmospheric gases. Thisproblem, of water permeability, however, is readily solved by providinga thin water impermeable coating on the side of the impregnated webmaterial that is to be in contact with the aqueous content of the can.This coating may be composed of any of the Wide variety ofwater-impermeable materials available, such as polyvinylidene, epoxy,polyester, oleo, and alkyd resins and metal foil, which would besuitable for use in contact with food. Preferably, a thin layer ofaluminum foil is used. This can be easily applied by laminating it tothe impregnated web material during the pressure curing of the resinimpregnated web. By use of this method the foil can be intergallylaminated to the resin impregnated web without the need for a separateadhesive.

The conversion of the cured resin impregnated web stock into canspresents certain problems which are not encountered in making of cansfrom the presently employed tinplate. Forming these webs into thedesired shapes for can bodies is much more difficult due to the factsthat (1) the impregnated web stock is considerably thicker, on the orderof 1.5 to 3 times as thick as the tinplate, and that (2) the stressstrain relationship of the impregnated webs is entirely different fromthat of tinplate. The modulus of elasticity of tin-plate is on the orderof 25,000,000 to 30,000,000 while that of impregnated Web stock suitablefor this invention ranges from about 500,000 to 2,500,000. As comparedto the resin-impregnated web stock of this invention then, tinplaterequires a much higher stress to produce a given deflection in the areaof non-deformable flexure. The stress-strain curve of tinplate,moreover, has a broad area from the point at which deformable flexurebegins until rupture occurs. This broad area of deformable fiexurepermits flat tinplate to be easily bent into the can body shape andpermanently deformed into that shape. This region of deformable flexure,however, is very limited in the stress strain-relationship of theresin-impregnated web stock at room temperature and permanentdeformation of the web stock is much more diflicult to achieve withoutrupturing the stock.

Due to the limited deformability of the impregnated web material at roomtemperature and to the interrelated factor of physical properties towithstand pressure forces, it has been found that the thickness of theimpregnated Web material must be controlled in its relation to thediameter of the can being made. The thickness of the can Wall should beless than of the can diameter and perferably in the range of A00 to Webthickness greater than of the can diameter will cause problems informing the can shape and in providing an economic package.

This invention may be better understood by referring to the drawingswherein FIGURES 1-4 are top cross-sectional views of cans illustratingmethods of forming a side seam.

FIGURES 5-8 are partial front elevations taken in section of methods offastening the can end.

Production of cured resin-impregnated Web stock satisfactory for use inthis invention can be prepared by a number of methods well known in theprior art. One method is to employ the current techniques used in thelaminating industry to produce fiat sheets of material which can then becut and formed into the can body. Obviously, it would greatly reducecosts to form a convolute or spiral tube from a non-curedresin-impregnated Web and cure the resin during the tube formation.However, such methods, except for those wherein the tube is subsequentlycured in a tube press for considerable time under substantial pressure,will not yield products which posses the necessary properties specifiedhereinabove. As currently available tube pressing methods are incapableof the large scale economical production needed for cans, forming of canbodies from flat pressed sheets is the preferred method.

In making the can body from flat, cured, resin-impregnated web stock, itis necessary that the stock be cut into the appropriate size for the canbody, the cut section formed into the cylindrical shape of the can body,and the edges permanently joined together creating a side seam 20.

The methods of forming side seams in tin cans are quite obviously notapplicable to the material of this invention since this material can notbe soldered. Joining the edges together may be simply accomplished,however, by applying an adhesive to the edges, bringing the edges intocontact with one another, and maintaining this contact until theadhesive is set. It will be obvious that the adhesive employed must beable to withstand the heat and moisture conditions of thermal processingwithout failure. Consequently, it is generally preferable to employ anadhesive of the thermosetting type. The melamine and epoxy resins havebeen found to be especially well suited for this use.

Simple butt glueing of the edges together will generally not providesuificient side seam strength in the can. It is consequently necessaryto employ other methods of glueing. Several satisfactory methods areshown in FIGURES 1 through 4 of the drawings. In FIGURE 1, a simpleoverlap seam is illustrated when the inner surface of one edge is gluedto the outer surface of the opposite edge of the can body. In FIGURE 2 amodified butt joint is shown which has a reinforcing strip 22 glued overthe butt joint. This particular joint may be further modified by use ofa tear string 24 which can be pulled to separate the reinforcing stripalong the seam line to provide for easy opening. The seams illustratedin FIGURES 1 and 2 have the undesirable characteristic of causing aprotrusion in the area of the seam due to the multiple thickness ofmaterial. This protrusion, which is also characteristic of the commontin can, often causes difiiculty in the opening of cans with the commontypes of can-openers and creates difficulties in providing a hermeticseal. This protrusion can easily be eliminated, however, by use of theconstructions shown in FIGURES 3 and 4 employing a beveled joint and aship lap joint respectively. As the beveled joint is more easilyprepared, and better controlled, it is the preferred type for use inthis invention.

Once the can body 21 has been formed, the completed can ready forfilling, is formed by attaching one or more end closures 25, 25. Theseend closures may be formed of metal, plastic or cured impregnated webstock similar to that employed in the can body. Many differentexpedients may be employed for attaching the end closures; a few ofwhich are illustrated in FIGURES 5 through 8. The presently preferredmethods of attaching the end closure 25 are shown in FIGURES 5 and 6employing a standard can end of tinplate or aluminum. Either of theseclosures can be made on double seaming equipment currently employed inmanufacturing tin cans.

In FIGURE the gasket material employed on the standard can ends ofordinary tin cans is replaced by an adhesive 26 such as a thermosettingepoxy resin. The end is placed on the can body and a so called falsedouble seam made by folding the edges of the end under adjacent portionswithout distributing the edge of the can body. While it is not necessaryto make the false seam, this provides several distinct advantages. Thefalse double seam not only maintains the closure in place during settingof the adhesives but also permits the use of presently available closingmachinery without major modification.

The method of attaching the can end shown in FIG- URE 6 is similar tothat in FIGURE 5 except that the terminal edge portion of the can bodyis flanged prior to attachment of the lid and is mechanicallyinterlocked with the can end during double seaming in the same manner asis commonly used on standard tin cans. Utilizing this method ofattaching the can end, an adhesive need not be employed although it ispreferable to do so or to employ a gasketing material similar to thatemployed in metal cans. It will be obviously that the bending of theedge of the can body through 180 at the very small radius involvedplaces a severe strain on the cured resin-impregnated web stock employedin the can body. In fact, it is very interesting that, due to the highrigidity and limited deformability of the can body stock of thisinvention, such an interlocking arrangement can be made without ultimatefailure of the can body along the bend. This is particularly true whenit is considered that this bending involves compound curvature of thematerial. However, it has been found that some cured resin-impregnatedpaper webs will undergo such compound curvatures without detrimenalresults by properly controlling the manufacture of the curedresin-impregnated web stock. Of primary importance in accomplishing thefianging of the can body is the type of resin employed. For satisfactoryfianging without cracking at the fold line, it is necessary to useeither a highly plasticized resin or one having relatively highdistortion. characteristics at elevated temperatures above thetemperature to be employed in thermal processing. In'the latter casefianging is easily accomplished at an elevated temperature of about 300to 350 F. The plasticizers used to develop the necessary bendingcharacteristics may be either external, i.e., those which do notactually enter the resin reaction, or internal, i.e., those which areintegrally reactive parts of the resin. Preferably internalplasticization should be employed due to the detrimental effects onstrength and moisture resistance generally caused by the commonlyemployed external plasticizers.

A method of internal plaseticization which has proved to be extremelyeffective with phenolic resins has been to utilize a phenol having analkyl group attached in the manufacture of the phenolic resin. It isgenerally preferable not to employ such modified phenols as the solesource of phenolic materials due to the increase in cost withoutsubstantial improvement in plasticity after a level of 50% alkylatedphenol has been reached. To achieve significant improvement inplasticity at least 10% of the phenolic material used in making theresin should be of the alkylated type. Suitable alkylated phenols arethose which contain a side chain of from about 4 to carbon atoms.Particularly suitable have been those having side chains in the middleof this range namely octyl or nonylphenol.

An additional factor which influences the ability of the can stock towithstand the deformation during interlocking with the can end isquantity of resin employed. Contrary to expectation, the greater thequantity of resin employed in the cured web stock, the easier it will beto form such an interlock. Consequently, it is preferred practice whenemploying the closure shown in FIGURE 6 that the resin loading beincreased to a level between about to Other well known methods ofsecuring the can end 25 to the body such as those in FIGURES 7 and 8 maybe employed.

The following examples illustrate the methods of manufacturing curedresin-impregnated web stock and the conversion thereof into cans.

Example 1 A 195 lb./3000 sq. ft. paper web was impregnated with a resinvarnish and dried to provide a ratio of :28z8 parts by weight of paper,phenolic resin, and volatiles, respectively. The phenolic resin wasprepared from phenols, formaldehyde, and sodium hydroxide at a moleratio of l:l.845:0.04. In preparing this resin a kettle was charged withthe following:

Pounds Nonyl phenol 17.25 Phenol, 92% U.S.P. 75.00 Flakeparaformaldehyde, 91% 49.41 Water 14.25 This mixture was preheated to atwhich time 2.60 lbs. of 50% sodium hydroxide was added in six equalportions at 5-minute intervals. After an additional 14 minutes ofcooking, the kettle temperature was raised from 160 F. to F. in 3minutes and kept at 180 F. for 22 minutes. The kettle was then cooled toroom temperature. The prepared resin contained 5.4% free formaldehydeand 63.4% solids.

The resin was then diluted to 47% solids with methanol, and the pH wasadjusted to 8.3 with the use of concentrated HCl.

The paper web was passed through a trough containing the above resinvarnish. A series of scraper bars and a set of squeeze rolls were usedto provide uniformity of impregnation. The amount of resin pickup wascontrolled by adjusting the web speed and scraper bars. The impregnatedweb was dried to the desired volatile content with the use of twosequential drying cabinets, the temperature of which was controlled at275 F.

The continuous dried resin-impregnated paper was cut into fiat sheets.Two of these sheets faced on one side with a thin sheet of aluminum foilwere pressed together to provide stock for the making of can bodies.

Pressing was accomplished at a temperature of 320 F. for 7.5 minutesemploying a pressure of 1500 p.s.i.

This laminated web stock was used in the fabrication of cans in thefollowing manner:

The laminated stock was cut into a rectangle of appropriate size for canbody construction. The two opposite sides of the can body blank whichform the side seam of the can were beveled with parallel slopes so thatthe width of the bevel was approximately 12 times the thickness of thelaminate.

An adhesive, which was composed'of a melamine formaldehyde resindissolved in water, was applied to both of the beveled edges.

The can body was formed by curling the body blank into a cylinder withthe aluminum foil surface on the inside and aligning the beveled edgesso that when bonded the thickness of the side seam was essentially thesame as that of the body material. The side seam was bonded by elevatingthe temperature to 320 F. while applying a pressure of 150 psi. to theoverlapping beveled area. This combination of heat and pressure effectedcure of the adhesive, permanently bonding the side seam.

A double seamer was used to attached the metal ends to the unfiangedcylinder by a false double seam. The same adhesive used for the sideseam was used to bond the metal end to the can body. This adhesive wasapplied inside the lip of the can end in place of the conventionallyused gasketing compound.

Size 303 x 406 cans fabricated in the above manner were pressure testedand easily withstood internal pressures up to 70 p.s.i.g. and externalpressures up to 20 p.s.i.g. with no structural failure. Cans were alsoemployed for the thermal processing of diced carrots and performed A 195lb./3 000 sq. ft. paper web was impregnated with a resin varnish toprovide a ratio of 100:28:8 parts paper, resin, and volatiles,respectively.

A phenolic resin was prepared which was prepared from phenol,formaldehyde and sodium hydroxide at a mole ratio of 1:1.845:0.04. Inpreparing this resin a kettle was charged with the following:

Pounds Phenol, 92% U.S.P 89.7 Flake paraformaldehyde, 91% 53.4 Water12.6

This mixture was preheated to 160 temperature of the kettle was raisedto 180 F. in 2 minutes and kept at this temperature for 16 minutes. Thekettle was then cooled to room temperature. The resulting resin varnishcontained 6.7% free formaldehyde and 61.2% solids.

This resin was mixed at a solids weight ratio of 1:1 with a kraft pinelignin. This varnish was then diluted wit-h methanol to a solids contentof 50% and the pH adjusted to 6.0 with the use of concentrated HCl.

The same method as described in Example 1 was used to impregnate thepaper web, press the laminate, and fabricate the can.

Flexure tests conducted on Web stock prepared in the above mannerrevealed an average modulus of elasticity after standard conditioning of1,700,000 p.s.i. and, after steam treatment, an average modulus ofelasticity of 1,420,000 p.s.i.

Example 3 A 150 lb./3000 sq. ft. paper web was impregnated with a resinvarnish to provide a ratio of 100240110 parts paper, epoxy resin, andvolatiles, respectively. The epoxy resin varnish was prepared bycombining 100 parts of a epoxide equivalent of 185-192, and 43 parts ofa reactive polyamide resin. These components were diluted to 30% solidswith methyl-ethyl ketone before treating.

Two sheets of the above impregnated paper web and a press time wasextended to minutes instead of 7.5 minutes.

Cans were fabricated from this high pressure epoxy laminate in the samemanner as described in Example 1.

Tests conducted on these cans revealed that internal and externalpressures of 50 p.s.ig. and 12 p.s.i.g., respectively, were withstoodwithout failure. No diificulty was encountered in retorting dicedcarrots in these cans.

Flexure tests conducted on web stock prepared in the above mannerrevealed an average after standard conditioning of 1,340,000 p.s.i. and,after steam treatment, an average modulus of elasticity of 1,000,000p.s.i.

We claim:

1. A can suitable for use in thermal processing of foods which comprisesa pre-stressed thin-wall cylindrical body member having a thicknessequal to from 1/100 to 1/200 of the diameter of said can and an endclosure hermetically sealed to one end of said body member, said bodymember being composed of a cylindrically formed single unitary web whoseopposite edges are adhesively secured together at an axially orientedseam extending from one end of the body member to the opposite end, saidweb comprising a non-woven cellulose fiber sheet impregnated with from15 to 60% by weight of a thermoset resin which has been cured underpressure in said sheet while said sheet is fiat and being characterizedby its ability to retain at least 65% of its modulus of elasticity inflexure following conditioning in saturated steam at 212 F. for 5minutes and by having a residual modulus of elasticity in flexure aftersuch conditioning of at least 500,000 p.s.i.

2. The can of claim 1 wherein said web retains at least of its modulusof elasticity in flexure after said conditioning and the residualmodulus of elasticity in flexure is at least 1,000,000 p.s.i.

3. The can of claim 1 wherein the interior surface of said body memberis coated with .a continuous waterimpervious coating.

4. The can of claim 3 wherein the water-impervious coating is aluminumfoil integrally laminated to the resin-impregnated web.

5. The can of claim 3 wherein the thickness of the can body at the seamformed by adhesively securing the opposite edges of the web together issubstantially the same as the thickness of said web.

6. A can suitable for use in thermal processing of foods which comprisesa pre-stressed thin-wall cylindrical body member and an end closurehermetically sealed to the opposite end, cellulose fiber sheetimpregnated with from 15 to 60% by Weight of a phenolic resin in whichfrom 10 to 50% of the phenolic component is an alkylated phenol thealkyl group of which contains from 4 to 15 carbon atoms, said resinhaving been cured under pressure in said sheet while said sheet prisinga non-woven of its modulus of elasticity in flexure followingconditioning in saturated steam at 212 F. for 5 minutes and by having aresidual modulus of elasticity in fiexure after such conditioning of atleast 500,000 p.s.i.

References Cited JOSEPH R. LECLAIR, Primary Examiner. FRANKLIN T.GARRETT, Examiner. V. A. TOMPSON, R. PESHOCK, Assistant Examiners.

1. A CAN SUITABLE FOR USE IN THERMAL PROCESSING OF FOODS WHICH COMPRISESA PRE-STRESSED THIN-WALL CYLINDRICAL BODY MEMBER HAVING A THICKNESSEQUAL TO FROM 1/100 TO 1/200 OF THE DIAMETER OF SAID CAN AND AN ENDCLOSURE HERMETICALLY SEALED TO ONE END OF SAID BODY MEMBER, SAID BODYMEMBER BEING COMPOSED OF A CYLINDRICALLY FORMED SINGLE UNITARY WEB WHOSEOPPOSITE EDGES ARE ADHESIVELY SECUCRED TOGETHER AT AN AXIALLY ORIENTEDSEAM EXTENDING FROM ONE END OF THE BODY MEMBER TO THE OPPOSITE END, SAIDWEB COMPRISING A NON-WOVEN CELLULOSE FIBER SHEET IMPREGNATED WITH FROM15 TO 60% BY WEIGHT